1. Field of the Invention
The present invention relates to a vibration actuator that effects a driving force by generating vibrations in an elastic member through an electro-mechanical conversion element.
2. Description of Related Art
Vibration actuators, such as ultrasonic actuators, are quiet, and have high torque, good control, and superior holding power. These actuators are classified generally as either linear or ring type. The ring type of actuators are used in AF motors of various mechanical devices, such as cameras.
FIG. 7 is an elevational view showing a conventional linear type ultrasonic actuator. A transformer 102, for use in excitation, is disposed at one end of a bar-shaped elastic member 101. Another transformer 103, for use in controlling vibrations, is disposed at the other end of the elastic member 101. Vibration elements 102a and 103a are connected to transformers 102 and 103, respectively. Alternating current is impressed from the oscillator 102b to the vibration element 102a used in excitation, which causes the bar-shaped elastic member 101 to vibrate. This vibration propagates across the bar-shaped elastic member 101 and becomes a progressive wave. The progressive wave drives a relative moving element 104 that is in pressure contact with the bar-shaped elastic member 101.
The vibration of the bar-shaped elastic member 101 is conveyed to the vibration element 103a via the vibration control transformer 103. The vibration energy is converted to electrical energy through the vibration element 103a. The vibrations are absorbed by the electrical energy consumed by a load 103b that is connected to the vibration element 103a. The reverberation of the end surface of the bar-shaped elastic member 101 is suppressed by the vibration control transformer 103. The transformer 103 thus prevents the generation of standing waves of the mode unique to the bar-shaped elastic member 101.
The length of the bar-shaped elastic member 101 must be within the moving range of the moving element 104 for the conventional linear type ultrasonic actuator of FIG. 7. Also, it is necessary to excite the entire bar-shaped elastic member 101. Consequently, conventional linear type ultrasonic actuators tend to be large in size. Additionally, a transformer 103 must be included for vibration control in order to prevent the generation of standing waves of unique mode.
In order to resolve the above and other problems, various self-advancing vibration actuators have been proposed, such as "longitudinal L1-bending B4 mode flat-plate motor," disclosed at pages 393-398, "222 piezoelectric linear motor for the purpose of optical pickup," that was included in "Lectures from the Fifth Electromagnetic Power-Related Dynamic Symposium." FIGS. 8A, 8B and 8C show different views of a "longitudinal L1-bending B4 mode flat-plate motor." FIG. 8A is a front view, FIG. 8B is a side view and FIG. 8C is a top view.
The elastic member 1 includes a base unit 1a that is formed into a flat, rectangular plate, and two driving force output members 1b and 1c. Members 1b and 1c project from one surface of the base unit 1a. Piezoelectric members 2 and 3 are bonded to another opposing surface of the base unit 1a. Piezoelectric members 2 and 3 include units that are excited by a driving voltage that is impressed thereon. The driving voltage generates the longitudinal vibration L1 mode and the bending vibration B4 mode in the elastic member 1.
The driving force output members 1b and 1c are disposed in an area that is the antinode of the bending vibration B4 mode in the elastic member. Output members 1b and 1c are pressed onto a relative moving element (not shown), such as a rail.
The ultrasonic actuator shown in FIGS. 8A, 8B and 8C is constructed so that the unique vibration frequencies of the longitudinal vibration L1 mode and the bending vibration B4 mode of the elastic member 1 are extremely close together. The two vibration modes are harmonized by alternating current having frequencies that are close to the two unique vibration frequencies impressed on the piezoelectric members 2 and 3. Elliptical motion is thus generated in the elastic member 1. The elliptical motion is output as thrust via the driving force output members 1b and 1c.
FIG. 9 is a conventional pressing mechanism for pressing an elastic member 1 toward a relative moving element 4, consistent with an ultrasonic actuator co-invented by Yoshiro Tomikawa and Sadayuki Ueha. The center of the elastic member 1, between the driving force output members 1b and 1c, is pressed toward the relative moving member 4 by the pressing mechanism 7. The pressing mechanism 7 includes a coil spring 6 and a pressing plate 5. The pressing plate 5 operates as a shock-absorber and is formed of felt.
The pressing mechanism 7 of FIG. 9 is constructed to press the center of the elastic member 1, between the driving force output members 1b and 1c, because that position is a common node for longitudinal and bending vibrations. Pressing the elastic member at the common node minimizes the resultant obstruction of the vibrations. The pressing mechanism 7, thus, minimizes the reduction in driving efficiency caused by pressing the elastic member 1.
However, the pressing plate 5 shown in FIG. 9 contacts the elastic member 1 through the piezoelectric members 2 and 3. This construction causes the vibrations of the elastic member 1 to be absorbed during pressing, which reduces driving efficiency.
Additionally, pressing the center of the elastic member 1 between the driving force output members 1b and 1c with the pressing mechanism 7 bends the elastic member 1, as shown in FIG. 10. The center of the elastic member 1 is at the apex of the bow. Small gaps are defined at the surfaces of contact between the driving force output members 1b and 1c and the relative moving element 4.
The driving force of the vibration actuators shown in FIGS. 8 and 9 is proportional to the product, F.times..mu., wherein F is the pressing force on the elastic member 1 and .mu. is the coefficient of dynamic friction between the driving force output members 1b and 1c and the relative moving element 4. Consequently, the pressing force F must be increased in order to increase the driving force of the vibration actuator. However, the elastic member 1 bends more, and the gaps between the driving force output members 1b and 1c and the relative moving element 4 increase, when the pressing force F increases. The increased gaps result in a reduction in the area of contact between the driving force output members 1b and 1c and the relative moving element 4. Thus, the sliding relationship between the elastic member 1 and the relative moving element 4 becomes unstable, and tends to shift the position of contact between these elements. This shifting of position causes problems, such as speed irregularities, and a shortening of the life of the vibration actuator based upon uneven wear on the bottom surfaces of the driving force output members 1b and 1c.
Furthermore, the pressing plate 5 of FIG. 9 includes a textile material, such as felt, to prevent short circuits. The inclusion of the textile material diminishes the weather-resistance of the apparatus. The pressing force of the pressing plate 5 also changes as it gets older, which reduces the reliability of the vibration actuator.